inverted LEDs

Discussion in 'General Electronics Chat' started by duxbuz, Apr 12, 2014.

  1. duxbuz

    Thread Starter Member

    Feb 23, 2014
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    Hi

    I am using this common anode 7 segment display:
    http://www.us.kingbright.com/images/catalog/spec/SA08-11EWA.pdf

    I have it connected to this decoder:
    http://www.farnell.com/datasheets/1700457.pdf

    I eventually realised that the logic is good it's just that all the led's are inverted!

    So a number 3 looks like a number 1, but aligned to the left!! The number 4 was just seemingly random leds, but I have realised that its just that the LEDs are inverted.

    I said the same thing twice. Anyway...

    Could anyone take a guess why all LEds are inverted?

    I can probe the pins and all the logic seems sound, highs and lows where expected.
     
  2. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    Look at page 1, lower right just above the chip outline. You are using LEDs with a chip designed to drive Liquid crystal displays. It will work but you need a transistor driver for each segment which will invert the output signal from the decoder. Look at the truth table for the various digits and this will become more clear.
     
  3. duxbuz

    Thread Starter Member

    Feb 23, 2014
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  4. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi dux,
    How is pin #6 , the PH, connected on the 4543 IC.?

    E
     
  5. duxbuz

    Thread Starter Member

    Feb 23, 2014
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    I have it with a 1.5k resistor pulled down to ground
     
  6. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    Try the LED display when Pin 6 is pulled High.

    Recheck the LED digits displayed.
     
    duxbuz likes this.
  7. duxbuz

    Thread Starter Member

    Feb 23, 2014
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    Legend thanks

    sorted
     
  8. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    Hi,
    This image shows the result of connection to High of the phase.

    You have used the internal XNOR to invert the output, the same as adding an external inverter.:)

    E
    EDIT: The Phase pin is normally switched by the Back Plane oscillator when driving an LCD, as the LCD segments require an 'AC' waveform drive.
     
  9. duxbuz

    Thread Starter Member

    Feb 23, 2014
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  10. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    The HC04 can sink/source a max of 25mA on any one pin, but you would have to ensure that the max Wattage did not exceed the rating of the IC.

    The ULN would easily sink the LED segment current, that would have been my choice.
    E
     
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Or change to a common cathode LED display instead?
     
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